U.S. patent number 6,370,954 [Application Number 09/659,168] was granted by the patent office on 2002-04-16 for semiconductor integrated inertial sensor with calibration microactuator.
This patent grant is currently assigned to STMicroelectronics S.r.l.. Invention is credited to Massimo Garavaglia, Gianluca Tomasi, Benedetto Vigna, Sarah Zerbini.
United States Patent |
6,370,954 |
Zerbini , et al. |
April 16, 2002 |
Semiconductor integrated inertial sensor with calibration
microactuator
Abstract
An inertial sensor having an inner stator and an outer rotor
that are electrostatically coupled together by mobile sensor arms
and fixed sensor arms. The rotor is connected to a calibration
microactuator comprising four sets of actuator elements arranged
one for each quadrant of the inertial sensor. There are two
actuators making up each set. The actuators are identical to each
other, are angularly equidistant, and each comprises a mobile
actuator arm connected to the rotor and bearing a plurality of
mobile actuator electrodes, and a pair of fixed actuator arms which
are set on opposite sides with respect to the corresponding mobile
actuator arm and bear a plurality of fixed actuator electrodes. The
mobile actuator electrodes and fixed actuator electrodes are
connected to a driving unit which biases them so as to cause a
preset motion of the rotor, the motion being detected by a sensing
unit connected to the fixed sensor arms.
Inventors: |
Zerbini; Sarah (Fontanellato,
IT), Vigna; Benedetto (Pietrapertosa, IT),
Garavaglia; Massimo (Robecchetto, IT), Tomasi;
Gianluca (Vigevano, IT) |
Assignee: |
STMicroelectronics S.r.l.
(Agrate Brianza, IT)
|
Family
ID: |
8243580 |
Appl.
No.: |
09/659,168 |
Filed: |
September 11, 2000 |
Foreign Application Priority Data
|
|
|
|
|
Sep 10, 1999 [EP] |
|
|
99830566 |
|
Current U.S.
Class: |
73/514.01;
73/514.02 |
Current CPC
Class: |
G01P
15/097 (20130101); G01P 15/125 (20130101); G01P
21/00 (20130101); G01P 2015/0814 (20130101) |
Current International
Class: |
G01P
21/00 (20060101); G01P 15/08 (20060101); G01P
15/10 (20060101); G01P 15/097 (20060101); G01P
15/125 (20060101); G01P 015/00 () |
Field of
Search: |
;73/504.02,504.03,504.04,504.08,504.09,504.12,504.13,514.32,514.01,514.02 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0 840 092 |
|
May 1998 |
|
EP |
|
89/10567 |
|
Nov 1989 |
|
WO |
|
95/34798 |
|
Dec 1995 |
|
WO |
|
96/39615 |
|
Dec 1996 |
|
WO |
|
Primary Examiner: Moller; Richard A.
Attorney, Agent or Firm: Jorgenson; Lisa K. Tarleton; E.
Russell SEED IP Law Group PLLC
Claims
What is claimed is:
1. An inertial sensor integrated in a body of semiconductor
material, comprising: a stator element and a rotor element that are
electrostatically coupled together, said rotor element comprising a
mobile mass, and microactuator means integrated in said body of
semiconductor material, said microactuator means connected to and
coplanar with said mobile mass of said rotor element.
2. The inertial sensor of claim 1, wherein said mobile mass is free
to move in one direction and said microactuator means comprise at
least one first actuator element having at least one mobile
actuator arm that is integral with said mobile mass and at least
one first fixed actuator arm facing said mobile actuator arm, said
mobile actuator arm and said first fixed actuator arm carrying
respective mobile actuator electrodes and first fixed actuator
electrodes that are comb fingered together and extend in a second
direction substantially parallel to said one direction.
3. The inertial sensor of claim 2, wherein in that said mobile
actuator electrodes extend on both sides of said mobile actuator
arm and said microactuator means comprise a second fixed actuator
arm carrying a plurality of second fixed actuator electrodes which
are comb fingered with respective mobile actuator electrodes, said
first and second fixed actuator arms being set on opposite sides
with respect to a corresponding mobile actuator arm.
4. The inertial sensor of claim 3, wherein said mobile mass has an
annular shape; said mobile actuator arm extends radially outwards
from said mobile mass; said first and second fixed actuator arms
extend radially towards said mobile mass; said mobile actuator
electrodes extend in a substantially circumferential direction and
are set equidistantly apart from one another along said mobile
actuator arm; and said first and second fixed actuator electrodes
extend in a substantially circumferential direction.
5. The inertial sensor of claim 3, wherein said rotor element
moreover comprises a plurality of mobile sensor arms that extend
from said mobile mass, and said stator element comprises a
plurality of pairs of fixed sensor arms facing said mobile sensor
arms, each pair of fixed sensor arms comprising a first fixed
sensor arm and a second fixed sensor arm that are set on opposite
sides with respect to the corresponding mobile sensor arm.
6. The inertial sensor of claim 4, wherein the inertial sensor
constitutes an angular acceleration sensor.
7. The inertial sensor of claim 3, wherein said one direction is a
rectilinear direction, and said sensor constitutes a sensor of
rectilinear motion.
8. The inertial sensor of claim 3, further comprising a driving
unit having output terminals coupled to said fixed actuator arms
and a sensing unit having input terminals coupled to said fixed
sensor arms.
9. The inertial sensor of claim 2, wherein said microactuator means
comprise further actuator elements that are identical to said at
least one first actuator element, said at least one first actuator
element and further actuator elements being set symmetrically with
respect to said mobile mass.
10. The inertial sensor of claim 1, wherein said microactuator
means comprise at least one set of actuator elements, each set of
actuator elements comprising at least two actuators that are
identical to one another and angularly equidistant from one
another.
11. An inertial sensor system formed in a body of semiconductor
material, comprising:
a stator and a rotor electrostatically coupled together by mobile
sensor arms and fixed sensor arms;
a calibration device coupled to the rotor;
a drive unit coupled to the calibration device, the calibration
device and the drive unit configured to cause a preset motion of
the rotor; and
a sensing unit coupled to the stator and configured to sense the
motion of the rotor.
12. The system of claim 11, wherein the calibration device
comprises a microactuator comprising at least one first actuator
element having at least one mobile actuator arm that is integral
with the rotor and at least one fixed actuator arm facing the
mobile actuator arm, the mobile actuator arm and the fixed actuator
arm having mobile actuator electrodes and fixed actuator
electrodes, respectively, that are comb fingered together.
13. The system of claim 11, wherein the calibration device
comprises a microactuator that comprises four sets of actuator
elements arranged one for each quadrant of the inertial sensor.
14. The system of claim 13, wherein each set of the four sets of
actuator elements comprises first and second actuators that are
identical to each other and are angularly equidistant apart.
15. The system of claim 14, wherein each actuator comprises a
mobile actuator arm connected to the rotor and having a plurality
of mobile actuator electrodes, and a pair of fixed actuator arms
formed on opposite sides of the mobile actuator arm and having a
plurality of fixed actuator electrodes.
16. The system of claim 15, wherein the mobile actuator electrodes
and the fixed actuator electrodes are connected to the driving
unit, and the driving unit is configured to bias the mobile
actuator electrodes and the fixed actuator electrodes to cause the
preset motion of the rotor.
17. The system of claim 13, wherein the microactuator is coupled to
and coplanar with the rotor.
18. The system of claim 17, wherein the rotor comprises a mobile
mass that is free to move in a first direction, and the
microactuator comprises at least one actuator having at least one
mobile actuator arm that is integral with the mobile mass and first
and second fixed actuator arms facing the mobile actuator arm, the
mobile actuator arm and the first and second fixed actuator arms
carrying respective mobile actuator electrodes and fixed actuator
electrodes that are comb fingered together and extend in a second
direction substantially parallel to the first direction.
19. The system of claim 18, wherein the mobile mass has an annular
shape, and the mobile actuator arm extends radially outwards from
the mobile mass; the first and second fixed actuator arms extend
radially towards the mobile mass; the mobile actuator electrodes
extend in a substantially circumferential direction and are set
equidistantly apart from one another along the mobile actuator arm;
and the first and second fixed actuator electrodes extend in a
substantially circumferential direction.
Description
TECHNICAL FIELD
The present invention regards a semiconductor integrated inertial
sensor with calibration microactuator.
BACKGROUND OF THE INVENTION
As is known, the possibility of exploiting machinery and
manufacturing processes typical of the microelectronics industry
enables semiconductor integrated inertial sensors to be
manufactured at a low cost, at the same time guaranteeing high
reliability in terms of performance.
Although these inertial sensors are advantageous from various
points of view, they present the drawback that their calibration is
very complex, as well as costly, in that it is difficult to
calibrate them at a wafer level.
In addition, the inertial sensors thus obtained have an offset and
output/input sensitivity that depends upon the parameters of the
process of fabrication, and consequently must be suitably
calibrated.
In order to calibrate the sensor, one first known solution involves
shaking of the inertial sensor, already inserted in its own
package, on an electrodynamic or piezoelectric actuator validated
according to required standards. The choice of a particular type of
actuator is assessed on the basis of the range of the operating
frequencies of the inertial sensor that is to be calibrated. The
calibration curve that is obtained is then, generally, stored in a
memory device formed in the die in which the inertial sensor itself
is made. Even though this first known solution is advantageous from
various points of view, it presents the drawback that it is
extremely difficult to achieve at the wafer level.
A second known solution is described in U.S. Pat. No. 5,621,157,
which envisages integration on one and the same wafer of the
inertial sensor to be calibrated and of an electrostatic actuator,
which simulates the unknown inertial quantity to be measured, and
has the following characteristics:
it is linear in the voltage applied;
it is precise; i.e., its operation is practically independent of
the parameters of the integration process.
"Practically independent" means that the electrostatic actuator has
a configuration which is less sensitive than the inertial sensor is
to the variations in the integration process adopted for the
fabrication of the inertial sensor itself.
In addition, this second known solution is valid for all sensors,
whether open loop sensors or closed loop sensors.
More in detail, the method and device described in U.S. Pat. No.
5,621,157 are implemented by means of an inertial sensor comprising
one mobile electrode (rotor) and two fixed electrodes (stators),
underneath which is set a service electrode (actuator also referred
to as "ground plane"). By varying the voltage applied to the
service electrode and keeping the voltage applied to the mobile
electrode at a fixed value, a lateral force is produced that acts
upon the mobile electrode along a direction parallel to the plane
in which the service electrode lies. This lateral force is
independent of the distance between the mobile electrode and the
fixed electrodes, a distance which is markedly affected by the
variations in the process of fabrication of the inertial sensor,
and consequently it can be used as a reference force for the
calibration of the inertial sensor itself.
Although this second solution is advantageous from a number of
standpoints, it presents, however, the drawback that, at each
variation in the voltage applied to the service electrode, there is
produced on the mobile electrode, in addition to the lateral force,
also a vertical force in a direction orthogonal to the plane in
which the service electrode is set. In addition, the lateral force
has a value other than zero only when different voltages are
applied to the two fixed electrodes. Consequently, the method
devised and the device made according to this second known solution
are far from efficient in terms of conversion of electrical energy
into mechanical energy, and are valid only for certain electrical
configurations of the inertial sensor that is to be calibrated.
SUMMARY OF THE INVENTION
The technical problem that lies at the basis of the disclosed
embodiments present invention is that of creating a semiconductor
integrated inertial sensor with a calibration microactuator that is
able to overcome the limitations and drawbacks referred to above in
connection with the known art.
The technical problem is solved by an inertial sensor integrated in
a body of semiconductor material and having a stator element and a
rotor element that are electrostatically coupled together, the
rotor element having a mobile mass, and a microactuator integrated
in the body of semiconductor material, the microactuator connected
to and coplanar with the mobile mass of the rotor element.
In accordance with another aspect of the present invention, the
mobile mass is free to move in one direction and the microactuator
has at least one first actuator element having at least one mobile
actuator arm that is integral with the mobile mass and at least one
first fixed actuator arm facing the mobile actuator arm, the mobile
actuator arm and the first fixed actuator arm carrying respective
multiple actuator electrodes and fixed actuator electrodes that are
comb fingered together and extend in a direction substantially
parallel to the first direction.
In accordance with yet another aspect of the invention, the mobile
actuator electrodes extend on both sides of the mobile actuator
arm, and the microactuator includes a second fixed actuator arm
carrying a plurality of second fixed actuator electrodes that are
comb fingered with respective mobile actuator electrodes, the first
and second fixed actuator arms set on opposite sides with respect
to a corresponding mobile actuator arm.
BRIEF DESCRIPTION OF THE DRAWINGS
The characteristics and advantages of the inertial sensor according
to the embodiments invention will emerge from the ensuing
description, which is given purely to provide a non-limiting
illustration, with reference to the attached drawings, in
which:
FIG. 1 schematically shows a first embodiment of an inertial sensor
according to the present invention;
FIG. 2 shows an example of embodiment of a microactuator for
calibrating the inertial sensor of FIG. 1;
FIG. 3 shows the equivalent electrical diagram of the structure of
FIG. 1; and
FIG. 4 is a schematic representation of a second embodiment of the
inertial sensor according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1, the reference number 1 designates an inertial sensor,
for example an angular acceleration sensor, integrated in a
semiconductor material, namely, silicon.
More in detail, the inertial sensor 1 comprises an inner stator 2,
which is integral with a die 6 in which the sensor itself is made,
and an outer rotor 3, capacitively coupled to the stator 2.
The rotor 3 comprises a mobile mass 4, which is suspended and has a
basically annular shape, and a plurality of mobile sensor arms 5,
which extend radially towards the stator 2 starting from the mobile
mass 4, are identical to each other and are at the same angular
distance apart, and elastic suspension and anchorage elements
(springs 8) which elastically connect the mobile mass 4 to first
anchoring and biasing regions 30, through which the rotor 3 and the
mobile sensor arms 5 are biased at a biasing voltage Vr, typically
of the value of 1.5 V.
The stator 2 comprises a plurality of pairs of fixed sensor arms
9a, 9b, one for each mobile sensor arm 5 of the rotor 3, which
extend inside the mobile mass 4 itself, between the mobile sensor
arms 5, and are fixed to second anchoring and biasing regions 40.
The pairs of fixed sensor arms 9a, 9b are arranged in such a way
that one mobile sensor arm 5 of the rotor 3 is set between a first
fixed sensor arm 9a and a second fixed sensor arm 9b of a pair of
fixed sensor arms 9a, 9b.
Typically, the stator 2 and the fixed sensor arms 9a, 9b are
biased, through the second anchoring and biasing regions 40, at a
biasing voltage Vs, which assumes values of between 1.5 V and 2.2
V.
Consequently, in the presence of angular stresses, the mobile mass
4 and the respective mobile sensor arms 5 rotate, in a micrometric
way, either clockwise or counterclockwise, as indicated by the
double headed arrow R.
A pair of fixed sensor arms 9a, 9b and the respective mobile sensor
arm 5 set between them can be modeled as a capacitive divider made
up of two variable capacitors connected in series together, in
which the two outer plates are defined by the fixed sensor arms 9a
and 9b of the stator 2, and the inner plates are defined by the
mobile sensor arms 5 of the rotor 3. In addition, the capacitive
dividers made up of all the pairs of fixed sensor arms 9a, 9b and
of the respective mobile sensor arms 5 are connected together in
parallel, with the intermediate nodes of the dividers connected
together by means of the mobile mass 4. Consequently, the entire
inertial sensor 1 may be represented in the way shown in FIG. 3,
where the electrode SI represents the set of fixed sensor arms 9a
and makes up a first variable capacitor 31, and the electrode S2
represents the set of fixed sensor arms 9b and makes up a second
variable capacitor 32.
The inertial sensor 1 also comprises an integrated microactuator 12
connected to the rotor 3.
More in detail, as shown in FIGS. 1 and 2 the microactuator 12
comprises four sets 27 of actuator elements 13 connected to and
coplanar with the rotor 3. The sets 27 are arranged one for each
quadrant 14 of the inertial sensor 1 and are angularly
equidistant.
In particular, there are two actuator elements 13 for each set 27,
which are identical to each other, and each of which comprises a
mobile actuator arm 15 which is connected to the mobile mass 4 of
the rotor 3 and extends radially outwards, starting from the mobile
mass 4 itself. Each mobile actuator arm 15 carries a plurality of
mobile actuator electrodes 16, which extend on either side of the
respective mobile actuator arm 15 in a basically circumferential
direction, the mobile actuator electrodes 16 being positioned
equidistantly along the respective mobile actuator arm 15.
Each actuator element 13 moreover includes a pair of fixed actuator
arms 17a, 17b, which extend radially, each pair of fixed actuator
arms 17a, 17b being made up of a first fixed actuator arm 17a and a
second fixed actuator arm 17b which are set on opposite sides with
respect to the corresponding mobile actuator arm 15. The first
fixed actuator arm 17a carries a plurality of first fixed actuator
electrodes 19a, and the second fixed actuator arm 17b carries a
plurality of second fixed actuator electrodes 19b.
The fixed actuator arms 17a, 17b are constrained to third anchoring
and biasing regions 50, through which they are biased at a biasing
voltage Vb, which assumes values of between 1.5 V and 5 V.
The fixed actuator electrodes 19a, 19b extend in a basically
circumferential direction towards the respective mobile actuator
arm 15 and are interspaced or comb fingered with the mobile
actuator electrodes 16. In practice, the fixed actuator electrodes
19a, 19b and the respective mobile actuator electrodes 16, like the
fixed sensor arms 9a, 9b and the mobile sensor arms 5, the fixed
actuator electrodes 19a, 19b and the respective mobile actuator
electrodes 16 of each actuator element 13, may be modeled as a
capacitive divider made up of two capacitors connected in series
together, in which the two outer plates are defined by the fixed
actuator electrodes 19a, 19b, and the two inner plates are defined
by the mobile actuator electrodes 16. In addition, the capacitive
dividers made up of all the actuator elements 13 are connected
together in parallel, with the intermediate nodes of the dividers
connected together via the mobile mass 4. Consequently, the
microactuator 12 may be represented in the way shown in FIG. 3,
where the electrode S3 represents the set of fixed actuator
electrodes 19a and forms a first actuation capacitor 33, and the
electrode S4 represents the set of fixed actuator electrodes 19b
and forms a second actuation capacitor 34.
FIG. 3 moreover shows a calibration circuit for calibrating the
inertial sensor 1, which comprises a driving unit 20 having output
terminals 22 and 23 coupled, respectively, to the fixed actuator
arms 17a and 17b, and hence to the electrodes S1 and S2, and a
sensing unit 24 having input terminals 25 and 26 coupled,
respectively, to the fixed sensor arms 9a and 9b of the stator 2,
and hence to the electrodes S3 and S4. The driving unit 20, as
described more in detail in what follows, generates driving
voltages V1(t) and V2(t) oscillating in opposition with respect to
a constant mean value Vd(t).
Operation of the inertial sensor 1 is as follows:
The driving voltages V1(t) and V2(t), equal to
V1(t)=Vb+Vd(t), and
V2(t)=Vb-Vd(t)
where Vb is a constant biasing voltage and Vd(t) is an alternating
voltage, for example square wave or sinusoidal wave, are applied,
respectively, to the fixed actuator electrodes 19a and 19b by the
driving unit 20.
The voltages V1(t) and V2(t) alternately generate, on the mobile
mass 4, a transverse force proportional to the number of fixed
actuator electrodes 19a, 19b and to the number of interacting
mobile actuator electrodes 16. In addition, given that the voltages
V1(t) and V2(t) are in counterphase, this transverse force is
directed first in one direction and then in the opposite
direction.
In particular, this transverse force tends to move each mobile
actuator electrode 16 away from the fixed actuator electrodes 19a,
19b , with respect to which the mobile actuator electrode 16 has a
lower potential difference, and to bring the mobile actuator
electrode 16 closer to the fixed actuator electrodes 19b, 19a, with
respect to which it has a higher potential difference. In this way,
the mobile mass 4 undergoes a rotary motion having a twisting
moment .tau. proportional to the biasing voltage Vb of the fixed
actuator arms 17a, 17b and to the alternating voltage Vd(t),
according to the following relation:
.tau.=.alpha.*Vb*Vd(t)
where the parameter .alpha. is the precision of the microactuator
12 and depends upon the number of pairs of electrodes, their
thickness and relative distance, according to the following
relation:
where .epsilon..sub.0 is the electric constant, N is the number of
electrodes, R.sub.m is the distance of the electrodes from the
center of rotation, t is the thickness of the electrodes (which
coincides with the thickness of the polysilicon used for making
them on the wafer), and g is the distance between the
electrodes.
The twisting moment .tau. is independent of the relative
displacement between the fixed actuator electrodes 19a, 19b and the
mobile actuator electrodes 16 in that it depends only upon the
distance g of the pairs of mobile/fixed electrodes, which is
constant, and does not depend upon the area of mutual facing, which
is variable, given that the electrodes extend substantially
parallel to the direction of motion R of the rotor 3, and hence of
the mobile electrodes 15.
The twisting moment .tau., to which the mobile mass 4 is subjected,
thus determines a modulation in phase opposition of the
capacitances of the two variable capacitors 31, 32, the two outer
plates of which are defined by the fixed sensor arms 9a, and 9b of
the stator 2, and the two inner plates of which are defined by the
mobile sensor arms 5 of the rotor 3. Of these two variable
capacitors 31, 32, the one defined by the mobile sensor arms 5 and
by the fixed sensor arm 9a, 9b that is at a smaller distance makes
up the effective capacitor, which determines the generation of the
sensing signal that indicates the twisting moment .tau. to which
the mobile mass 4 is subjected.
This sensing signal is then sent to the input terminals 25 and 26
of the sensing unit 24, which uses it as a reference signal for
calibrating the inertial sensor 1.
The advantages that may be obtained with the inertial sensor
described herein are the following: In the first place, the
actuator elements 13 are defined on silicon together with the
mobile mass 4, and consequently do not require additional
fabrication phases. In addition, the inertial sensor 1 is more
efficient as regards the conversion of electrical energy into
mechanical energy, because the microactuator 12 does not generate
any force that acts perpendicularly on the mobile mass 4.
Furthermore, since the transverse force that acts on the mobile
mass 4 is independent of the biasing voltages applied to the rotor
3 and the stator 2 of the inertial sensor 1, calibration of the
inertial sensor 1 is independent of its own operating voltage. The
last two advantages described are very important in that they
render the inertial sensor 1 less costly in terms of energy and,
above all, render the circuit configuration of the microactuator 12
independent of the circuit configuration of the inertial sensor 1
(for example, sigma delta, frequency modulation). In addition, the
comb finger configuration chosen for the fixed actuator electrodes
19a, 19b and mobile actuator electrodes 16 is not affected by the
problem of electrostatic softening (i.e., reduction in the rigidity
of the system). Consequently, the characteristics of the actuator
are not modified, and the latter may exert a force independent of
the displacement.
Furthermore, for the fabrication of the inertial sensor 1 any type
of micromachining technology may be used (for example, surface or
epitaxial micromachining, metal electroplating, etc.).
Finally, it is clear that numerous modifications and variations may
be made to the inertial sensor described and illustrated herein,
all falling within the scope of the inventive idea as defined in
the attached claims and the equivalents thereof.
For example, the number of sets 27 of actuator elements 13 and the
number of actuator elements 13 in each set 27 could be different
from what has been described; in particular, it would be possible
to envisage even a single actuator element 13 connected to the
mobile mass 4 of the rotor 3, or else two actuator elements 13
could be envisaged, set on diametrically opposite sides of the
mobile mass 4.
In addition, the inertial sensor 1 may be of a linear type, as
shown in FIG. 4, in which the various parts of the inertial sensor
are indicated by the same reference numbers as those used in FIG.
1. In this case, the microactuator 12 is driven so as to impress on
the mobile mass 4 a vibratory motion along a direction Y, and the
mobile actuator electrodes 16 and fixed actuator electrodes 19a,
19b are parallel to the direction Y.
* * * * *